In the development history of 3rd Generation Partnership Project (3GPP) systems, different systems adopt different handover technologies in different scenarios. In a macro-cell-dominated mobile network, for example, an Enhanced Data Rate for Global System for Mobile Communications Evolution (GSM EDGE) Radio Access Network (GERAN), a main purpose of handover is to protect continuity of coverage. In such a network, handover adopts a so-called “hard handover” manner. Hard handover refers to that User Equipment (UE) is disconnected from an original serving cell before being connected to a new target cell. Such a handover technology has a defect that there may exist a short interruption process for user-plane transmission in a handover process. For a real-time service, this also means packet loss. If a user-plane interruption time becomes relatively long because of an excessively small intercell overlapping region and the like, a user may obviously feel this interruption, and for example, may hear obvious “clicks”.
When the 3GPP introduces a Universal Mobile Telecommunications System (UMTS), for improving a user experience, a “soft handover” technology is introduced. At first, mobile UE supporting soft handover is required to support a capability of keeping wireless connections with at least two cells. Before soft handover occurs, a network may connect the mobile UE with a target cell at first. Then, the network notifies the mobile UE of being handed over to this target cell. At the same time of handover to the target cell, the mobile UE may be required to be disconnected from a source target cell, and may also select to keep a wireless connection. Since wireless interface communication is kept all the time in this process, there is theoretically no user-plane interruption time. This is why such handover is called as “soft handover”. If the source serving cell and the target cell are located within the same eNB, such handover is called as “softer handover”. This is because message exchange and data packet forwarding processes between the source cell and the target cell are both eliminated. Therefore, the handover process is faster.
When the 3GPP develops a Long Term Evolution (LTE) system, for improving spectral efficiency, more and more microcells are adopted in engineering. There are two relatively significant differences between these microcells and macro-cells, one is that relatively high spectrum resources, for example, 2.4 GHz, are adopted, and the other is that their coverage is obviously narrowed, probably to be one tenth of the macro cells only. These microcells mainly function to absorb uplink and downlink service traffic of the mobile UE. Therefore, under a normal circumstance, these microcells have geographically overlapped coverage regions with the macro-cells, that is, the microcells are within coverage of the macro-cells under the normal circumstance. When moving among the macro-cells, the mobile UE may penetrate through a considerable number of microcells. If the handover technology is also adopted for penetrating through the microcells, a user-plane interruption may be generated. In addition, for the LTE system, signaling interaction with a core network is inevitable for both S1 handover and X2 handover. Excessive handover may cause a signaling storm of the core network. Under such a circumstance, the 3GPP introduces two technologies to eliminate negative influence brought by these microcells. When a backhaul between eNBs is an ideal backhaul, a Carrier Aggregation (CA) technology is adopted. When the backhaul between the eNBs is a non-ideal backhaul, a Dual Connectivity (DC) technology is adopted. If the CA technology is adopted, when the mobile UE penetrates through the microcells, handover is turned into a Secondary Cell (SCell) addition and deletion process. Since the UE always keeps being connected with a macro cell serving as a Primary Cell (PCell), SCell addition and deletion may not bring any user-plane interruption but cause traffic fluctuation of an air interface service. CA is configured on the premise that all Component Carriers (CCs) are controlled by a scheduler. Moreover, when the CCs are distributed at different stations, these stations are connected together through ideal backhauls. If the backhaul between the eNBs is non-ideal for some reasons, for example, a cost problem, the DC technology is required to be adopted. After DC is configured, the UE may be configured with three bearer manners, i.e. a Master Cell Group (MCG) bearer, a Secondary Cell Group (SCG) bearer and a split bearer. The MCG bearer refers to a radio bearer independently configured on an eNB. The SCG bearer refers to a bearer independently configured on a Secondary eNB (SeNB). The split bearer refers to a radio bearer configured on both the eNB and the SeNB. When the UE penetrates through coverage of the SeNB, an SeNB addition, deletion or SeNB changing process may be generated. For the split bearer, a bearer effect similar to that under CA may be achieved, that is, there is no user-plane interruption, but traffic fluctuation may be brought. However, for the SCG bearer, an effect similar to hard handover may be achieved.
CA and the split bearer under DC have achieved an effect similar to “seamless handover” from the angle of a user plane when the PCell is not changed. However, when the PCell is required to be changed or the SCG bearer requires replacement of the SeNB, the same problem of abovementioned “hard handover” or “soft handover” and “softer handover” may also appear on the user plane, i.e. the problem of user-plane interruption. The most important reason for the user-plane interruption is a security measure of the 3GPP.
For data security, ciphering and deciphering processes for user-plane data of the 3GPP systems on a wireless interface are required. In case of Radio Resource Control (RRC) signaling, an integrity protection process is also required. For the UMTS, security related configuration and calculation is implemented in a Radio Network Controller (RNC) above a Media Access Control (MAC) protocol layer. In the LTE system, security related configuration and calculation is implemented on a Packet Data Convergence Protocol (PDCP) layer. Once a security parameter is required to be reconfigured because of handover or SeNB changing, (a) PDCP and/or Radio Link Control (RLC) layer(s) of a related radio bearer are/is required to be reestablished, and a MAC layer and a Physical (PHY) layer may be reset. Moreover, a universal flow, usually a random access process, is required to be introduced to synchronize the security parameter, namely ensuring complete consistency in time when the mobile UE and the eNB adopt a new security parameter for a user plane of a certain specific radio bearer. Such a control-plane synchronization process and user-plane reestablishment/resetting process finally cause the user-plane interruption.
A trend of development of a cellular communication system in the future is that spectrums of low frequency bands become more and more valuable and expensive and microcells may use more spectrums of high frequency bands, for example, 3.5 GHz. With 6 GHz as a boundary, micrometer waves of more than 6 GHz will get increasingly popular. This narrows coverage of the microcells more because of spectrums. On the other hand, due to rapid development of technologies such as the mobile Internet and the Internet of things, a user connection number and user traffic in a unit area geometrically progressively increase. For increasing network traffic, it is feasible to arrange more microcells in a unit area. For reducing interference between the microcells, generated power of the microcells must be controlled within a certain range. This makes a future cellular network mainly characterized to be a high-density microcellular network. Under such a circumstance, a function of macro-cells may be degenerated mainly to bear control-plane signaling. In many indoor scenarios, for example, a gymnasium and a shopping mall, there may appear a network layout with only microcells. Compared with a prior macro-cellular homogeneous network or macro-cellular and microcellular homogeneous network, UE may move more frequently among the microcells.
Therefore, frequent replacement of a PCell may not be avoided even under the circumstance that CA or DC is configured. A user-plane interruption caused by frequent replacement of the PCell may seriously affect traffic control of a Transmission Control Protocol (TCP) layer, and in case of TCP congestion or TCP packet loss, TCP traffic control windows are rapidly reduced to make traffic control of the TCP layer zigzag.